The present invention relates to a color cathode ray tube apparatus and, more particularly, to a color cathode ray tube apparatus for reducing the elliptic distortion of a beam spot at the periphery of a screen and displaying a high-quality image.
A color cathode ray tube apparatus has an envelope made up of a panel and funnel. The funnel incorporates in its neck an electron gun assembly for emitting three electron beams, i.e., a center beam and a pair of side beams that pass through the same horizontal plane. A deflection yoke for forming a nonuniform magnetic field for deflecting the three electron beams is mounted on the funnel. The nonuniform magnetic field is formed from a pincushion type horizontal deflection magnetic field and barrel type vertical deflection magnetic field.
Three electron beams emitted by the electron gun assembly are focused on a phosphor screen while being converged to the entire surface of the phosphor screen formed on the inner surface of the panel through a shadow mask by the nonuniform magnetic field. Then, a color image is displayed.
The color cathode ray tube apparatus adopts, e.g., a BPF (Bi-Potential Focus) DACandF (Dynamic Astigmatism Correction and Focus) type electron gun assembly.
As shown in FIG. 1, this electron gun assembly has three cathodes K aligned in a line, and a first grid G1, second grid G2, third grid G3 made up of a first segment G31 and second segment G32, and fourth grid G4 which are sequentially laid out in a tube axis direction from the cathode K to the phosphor screen. Each grid has three electron beam apertures formed in correspondence with the three cathodes K.
In the electron gun assembly, the cathode K receives a voltage prepared by superposing a video signal on a reference voltage of 150 V. The first grid G1 is grounded, and the second grid G2 receives a voltage of about 600 V. The first segment G31 of the third grid G3 receives a voltage of about 6 kV, and the second segment G32 thereof receives a variable voltage prepared by superposing a parabolic voltage on a reference voltage of about 6 kV. This parabolic voltage increases with an increase in electron beam deflection amount, and maximizes for the maximum deflection amount, i.e., in deflecting an electron beam to the corner of the phosphor screen. The fourth grid G4 receives a voltage of about 26 kV.
The cathodes K, first grid G1, and second grid G2 constitute an electron beam generator for generating an electron beam and forming an object point with respect to a main lens (to be described later). The second grid G2 and the first segment G31 of the third grid G3 constitute a pre-focusing lens for preliminarily focusing the generated electron beam. The second segment G32 of the third grid G3 and the fourth grid G4 constitute a BPF type main lens for finally accelerating and focusing the preliminarily focused electron beam on the phosphor screen.
When the electron beam is deflected to the corner of the phosphor screen, the potential difference between the second segment G32 and fourth grid G4 minimizes to minimize the power of the main lens. At the same time, the maximum potential difference occurs between the first and second segments G31 and G32 to form a quadrupole lens for horizontally focusing an electron beam and vertically diverging it. The power of the quadrupole lens at that time is maximum.
When the electron beam is deflected to the corner of the phosphor screen, the distance from the electron gun assembly to the phosphor screen becomes maximum, and the distance from the object point to the image point becomes longer. An increase in distance from the object point to the image point is compensated by weakening the power of the main lens. The deflection aberration of the nonuniform magnetic field formed by the deflection yoke is compensated by the action of a quadrupole lens formed between the first and second segments G31 and G32.
To improve the image quality of the color cathode ray tube apparatus, the focusing characteristic and beam spot shape on the phosphor screen must be improved. Particularly in an in-line type color cathode ray tube apparatus for emitting three electron beams in a line, a beam spot 1 at the center of the screen can be made circular, as shown in FIG. 2. However, a beam spot 1 at the periphery extending from the end of the horizontal axis (X-axis) to the end of the diagonal axis (D-axis) elliptically distorts (vertically collapses) and causes a blur 2 owing to the deflection aberration.
The blur 2 of the beam spot 1 can be eliminated, as shown in FIG. 3, by adopting the DACandF method of dividing a low-voltage electrode forming a main lens into a plurality of segments, like the third grid G3 of the electron gun assembly. However, the elliptic distortion of the beam spot 1 at the periphery of the screen cannot be eliminated. This elliptic distortion interferes with the electron beam aperture of the shadow mask to generate moire, which makes it difficult to see the display contents.
The vertical collapse of the beam spot 1 at the periphery will be explained with reference to optical models shown in FIGS. 4 and 5. In a no-deflection state in which an electron beam is focused on the center of the screen, an electron beam 8 generated by the electron beam generator is preliminarily focused by a pre-focusing lens, and focused on a phosphor screen 5 by a main lens 4. In a deflection state in which an electron beam is deflected to the periphery of the phosphor screen, the electron beam 8 is preliminarily focused by the pre-focusing lens, passes through a quadrupole lens 6, and deflected by a deflection magnetic field 7 having a quadrupole component while being focused on the phosphor screen 5 by the main lens 4. Then, the electron beam 8 is focused on the phosphor screen 5.
In general, the beam spot size on the screen depends on a magnification M. The magnification M is given by the ratio xcex10/xcex1i of a divergent angle xcex10 and incident angle xcex1i of the electron beams 8. Letting Mh be the horizontal magnification, Mv be the vertical magnification, xcex10h be the horizontal divergent angle, xcex1ih be the horizontal incident angle, xcex10v be the vertical divergent angle, and xcex1iv be the vertical incident angle, the horizontal and vertical magnifications Mh and Mv are given by
Mh=xcex10h/xcex1ih
Mv=xcex10h/xcex1iv
If
xe2x80x83xcex10h=xcex10v
the above components satisfy
xcex1ih=xcex1iv
Mh=Mv
in the no-deflection state shown in FIG. 4.
The beam spot at the center of the screen becomes circular. To the contrary, in the deflection state shown in FIG. 5, the above components change to
xcex1ih less than xcex1iv
Mh greater than Mv.
The beam spot extends along the D-axis at the periphery.
As described above, to improve the image quality of the color cathode ray tube apparatus, the focusing characteristic and beam spot shape on the phosphor screen must be improved.
As for the focusing characteristic and beam spot shape, the conventional BPF DACandF type electron gun assembly changes the power of the main lens along with changes in electron beam deflection amount. In addition, the electron gun assembly forms a dynamically changing quadrupole lens to eliminate any vertical blur of the beam spot caused by the deflection aberration and focus the electron beam on the entire screen.
However, the elliptic distortion of the beam spot at the periphery cannot be eliminated. This elliptic distortion may interfere with the electron beam apertures of the shadow mask to generate moire, degrading the display quality.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a color cathode ray tube apparatus for reducing the elliptic distortion of a beam spot on the entire screen and displaying a high-quality image.
According to the present invention, there is provided a color cathode ray tube apparatus comprising an electron gun assembly having a main lens which is made up of at least a focusing electrode and anode electrode, and accelerates and focuses an electron beam on a phosphor screen, and a deflection yoke for generating a deflection magnetic field for deflecting the electron beam emitted by the electron gun assembly, wherein the electron gun assembly has at least one additional electrode located along an equipotential plane of a potential distribution formed between the focusing electrode and anode electrode forming the main lens, in a no-deflection state in which the electron beam is focused on a center of the phosphor screen, the additional electrode receives a voltage of a predetermined level corresponding to a potential of the equipotential plane on which the additional electrode is located, and in a deflection state in which the electron beam is deflected to a periphery of the phosphor screen, letting Vf be an application voltage of the focusing electrode, Eb be an application voltage of the anode electrode, and Vs be an application voltage of the additional electrode, a value
(Vsxe2x88x92Vf)/(Ebxe2x88x92Vf)
changes with an increase in electron beam deflection amount, while the additional electrode forms an electron lens having different horizontal and vertical focusing powers.
According to the present invention, there is provided a color cathode ray tube apparatus comprising an electron gun assembly having a main lens which is made up of at least a focusing electrode and anode electrode, and accelerates and focuses an electron beam on a phosphor screen, and a deflection yoke for generating a deflection magnetic field for deflecting the electron beam emitted by the electron gun assembly, wherein the electron gun assembly has at least one additional electrode located along an equipotential plane of a potential distribution formed between the focusing electrode and anode electrode forming the main lens, in a predetermined deflection state in which the electron beam is deflected, the additional electrode receives a voltage of a predetermined level corresponding to a potential of the equipotential plane on which the additional electrode is located, and in a deflection state in which the electron beam is deflected to a periphery of the phosphor screen, letting Vf be an application voltage of the focusing electrode, Eb be an application voltage of the anode electrode, and Vs be an application voltage of the additional electrode, a value
(Vsxe2x88x92Vf)/(Ebxe2x88x92Vf)
changes with an increase in electron beam deflection amount, while the additional electrode forms an electron lens having different horizontal and vertical focusing powers.